Methyl ethers of polyalkoxylated polyols for removing acidic gases from gases

ABSTRACT

Polymethyl ethers of a polyether polyol, the polyether polyol having the formula WHERE R is the residue of an organic compound having three to six hydroxyls, A is 1 to 6, B is 0 to 5 and A + B equals 3 to 6 and Y is 1 to 15 GROUPS OR MIXTURES THEREOF. Ethoxylated glycerol trimethyl ether is disclosed. The compounds are useful for extracting acid gases such as CO2 and H2S from natural gas.

Umted States Patent [151 3,653,183

Sanders et al. 1 Apr. 4, 1972 [54} METHYL ETHERS OF lgfrsllley etlal. Foc eta ii iQ I C 2,951,094 8/1960 Hefner ..260/6l5 B REMOVING C3,362,133 1/1968 Kutsher et al ..55/73 x GASES 72 Inventors: Herbert L.Sanders, Skokie, 111.; Robert A. ggfgfgf fgl t afgfizfs fig sm Braunwanhboth of Attorney-Merriam, Marshall, Shapiro& Klose [73] Assignee:Northern Petrochemical Company, Des TRACT Plames Polymethyl ethers of "1polyether polyol, the polyether polyol [22] Filed: Feb. 12, 1970havingthe formula [21] Appl. No.: 11,003 (YOH) R 52 U s c1 55/56 55/73\(OH)" B01; 53/00 where R is the residue of an organic compound having 3to 6 58 Field 4155; 55/44 53 55 56 68 73- hydmxyls' A is l B is 0 to 5and B equals 3 6 260/615]; endY1sltol5 H3 CH- CH O or CH HO [56]References Cited groups or mixtures thereof. Ethoxylated glyceroltrimethyl UNITED STATES PATENTS ether is disclosed. The compounds areuseful for extracting 2 552 52s 5/1951 De Groote ..260/6l5 B acid gasesSuch as CO2 and from natural 2,649,l66 8/1953 Porter et al, ..55/68 X 8i N0 Drawings M ETHYL ETHERS OF POLYALKOXYLATED POLYOLS FOR REMOVINGACIDIC GASES FROM GASES This invention is concerned with novelderivatives of polyether polyols which remove acid gases from admixturewith non-acidic gases. The invention is also concerned with an improvedprocess for the removal of carbon dioxide and/or hydrogen sulfide from agaseous mixture of hydrocarbons and/or other nonacidic constituentscontaining said acid gases, using as a selective solvent a mixture ofmethyl ethers of a polyalkoxylated polyol.

The acid gas content of natural gas varies between broad limits,depending on the field from which it is produced. Natural gas from somesources contains undesirably high concentrations of acid gases such ashydrogen sulfide and carbon dioxide. Before this gas can be sold, theacid gas content must be reduced to an acceptable concentration. Variousmethods to remove acid gases from natural gas have been proposed. Oneapproach which is used is to contact the natural gas with a chemicalwhich reacts with and removes acid gases in chemically combined form.For example, hot potassium carbonate, monoethanolamine, anddiglycolamine have been used for removing carbon dioxide from naturalgas. Because of the expense of regenerating a chemically reactivepurification solvent by heating, the cost of treating natural gaseswhich have high acid gas contents with a reactive solvent becomes veryhigh.

A second approach to acid gas removal is the use of a physical solventwhich has selective solvency for acid gases rather than the hydrocarbongas content. Examples of such physical solvents which have been usedcommercially are propylene carbonate and polyethylene glycol dimethylether. The solubilities of acid gases in the solvent is a function ofthe partial pressure of the acid gases, and of the total pressure of thesystem. No elaborate solvent regeneration process is needed since theacid gases are simply released when pressure is reduced. While there areuseful solvents for this purpose, additional solvents are needed so thatthe choice of solvent for a particular use is not restricted bycommercial availability and economics.

According to the present invention, there are provided novel methylethers of polyether polyols useful as solvents for removing acid gasessuch as carbon dioxide and hydrogen sulfide from natural gas and othergaseous lower alkane hydrocarbons as well as from hydrogen and nitrogen.

The methyl ethers of polyether polyols provided herewith contain fromabout three to six methoxy groups and are derivatives of polyetherpolyols having at least three and up to six hydroxyl groups.

The polyether polyols used in producing the polymethyl ether derivativescan be represented as follows:

where R is the residue of an organic compound containing therein threeto six active hydrogen atoms present in hydroxyl groups, A is a numberfrom 1 to 6, B is a number from to 5 and A B equals 3 to 6, and yrepresents from 1 to groups or such groups in admixture in random orblock arrangement. The polyether polyols are thus polyoxyalkylenederivatives of polyols having three to six hydroxyls and thepolyoxyalkylene groups are polyoxyethylene, polyoxypropylene or mixturesof polyoxyethylene polyoxypropylene groups. Typical of the organicresidues which Y can represent are those from polyols such as glycerol,trimethylolpropane, pentaerythritol, dipentaerythritol, sorbitol and1,2,6-hexanetriol.

More specifically, some of the polyether polyols useful in thisinvention are:

polyoxyethylene adducts of glycerol,

polyoxypropylene adducts of glycerol,

polyoxyethylene adducts of trimethylolpropane,

polyoxypropylene adducts of trimethylolpropane,

polyoxyethylene adducts of pentaerythritol, polyoxypropylene adducts ofsorbitol, polyoxypropylene-polyoxyethylene adducts of glycerol andpolyoxypropylene-polyoxyethylene adducts of trimethylolpropane Thosecompounds containing both polyoxypropylene and polyoxyethylene groupscan have such groups present as blocks or randomly located. Furthermore,all of the polyoxyalkylene groups can be positioned on all, one, or anynumber less than all, of the hydroxyl groups of the polyol base.Usually, however, the polyether polyol is prepared by alkoxylating allthe hydroxyl groups of the polyol.

The polyether polyols are readily prepared by procedures disclosed inthe art. Thus, to alkoxylate all the hydroxyl groups on a polyol, analkylene oxide or mixture of alkylene oxides can be reacted with thepolyol at an elevated temperature and pressure in the presence of analkaline catalyst such as sodium alkoxide or sodium hydroxide. An amountof alkylene oxide is used with a given polyol to produce a polyetherpolyol of suitable average molecular weight. The addition of ethyleneoxide or propylene oxide to a polyol produces a complex mixture ofadducts which vary both in the number of oxide units attached to eachmolecule and in the position at which the oxide units are attached tothe molecule. Since the polyether polyol is a mixture of compounds ofdifferent molecular weights, it is assumed that the average molecularweight of these compounds is the molecular weight of the product. It canbe calculated from the amount of alkylene oxide added and/or thehydroxyl number of the adduct.

There are many US. Pat. Nos. disclosing the preparation of polyetherpolyols which, in a suitable molecular weight, can be used in thesubject invention, amongst which are: 2,552,528; 2,597,204; 2,626,911;2,673,882; 2,674,619; 2,733,272; 2,866,774; 2,927,918; and 2,948,757.Although some of the polyether polyols disclosed in these patents havehigher molecular weights than would those used in this invention, it issimple to make the lower molecular weight compounds by using lessalkylene oxide in the reaction to react with the polyol.

To produce polyether polyols in which one or any number less than allthe hydroxyl groups are alkoxylated with an alkylene oxide, it isadvisable to first convert those hydroxyl groups which it is not desiredto alkoxylate to methyl ethers. The methyl ether groups are nonreactiveto alkylene oxide which thus reacts only with the remaining freehydroxyl groups. Such reactions are well known in the art.

Conversion of a polyether polyol to the polymethyl ether is readilyeffected by reacting it with sodium hydroxide and methyl chloride,although it is possible to use metallic sodium in place of the sodiumhydroxide, and/or other methyl halides or dimethyl sulfate in place ofmethyl chloride. in any event, the methyl ether formation is accompaniedby the formation of a byproduct salt which is separated from theproduct. The salt can be separated by conventional means such asfiltration, decantation, extraction, and/or distillation. In some cases,it is advantageous to conduct the methylation in two or more steps withsalt separation after each step.

The polymethyl ether derivatives of the polyether polyols providedbroadly according to this invention can be represented by the formulawherein C is a number from to 3, D is a number from 1 to 6, E is anumber from 0 to 3, F is a number from 0 to 5 and C+D+E+F equals 3 to 6and Y represents from 1 to groups or such groups in admixture in randomor block arrangement and R has the significance previously assigned.

in general, the properties of the compounds of this invention such aslow viscosity, low vapor pressure and high solvency for acid gases areoptimized by a high degree of methylation, although for some purposes itis not necessary to completely methylate the polyether polyol. Productswhich are generally most suitable for use as acid gas solvents are thosehaving at least 70 percent and advisably 90 percent or more of thehydroxyl groups of the polyether polyol converted to methyl ethergroups. Products which have an average molecular weight between about150 to 800, and advisably about 200 and 600, generally have the mostdesirable properties for use as solvents for removing acid gases fromnatural gas and other gaseous hydrocarbons. Particularly useful solventsprovided herewith are those of the formula wherein X, Y and Z representfrom O to 15 groups or such groups in admixture and the sum of X Y Z isl to 15, and A is l to 4. Furthermore, those where A is l and X, Y and Zare approximately the same number are generally especially useful.

The treatment of natural gas, gaseous lower alkane hydrocarbons,hydrogen and nitrogen containing one or more acid gases with thesolvents of this invention is carried out using conventional absorptionprocedures, wherein the gaseous mixture is contacted with the solventunder pressure in a countercurrent absorption tower in a continuous flowmethod. The acid gasenriched solvent is continuously withdrawn from theabsorption tower and is introduced into a flash chamber to remove theabsorbed acid gases by reduction of pressure. A vacuum flash or airstripping column may also be used as a final step to further reduce theacid gas content of the solvent. The regenerated solvent is thenre-cycled through the absorption tower where it is used again.

The extraction process is best carried out at temperatures within therange from about to 120 F. although higher or lower temperatures may beutilized. The minimum temperature at which a solvent may be used isgoverned by the increased viscosity of the solvent at low temperatures;the maximum temperature at which a solvent may be used is governed bythe lower acid gas solubility at higher temperatures, as well as theincreased vapor pressure of the solvent at high temperatures.

The following examples will serve to illustrate the preparation of someof the novel compounds of this invention, and to demonstrate theexperimentally determined utility of the compounds as solvents for acidgases.

EXAMPLE 1 Preparation of Ethoxylated Glycerol Trimethyl Ether To a2-liter stainless steel stirred pressure reactor was charged 492 g. ofglycerol and 1.0 g. of 50 percent sodium hydroxide. The charge washeated to 280 F. at which temperature a total of 608 g. of ethyleneoxide was added over a period of 3 hours. The resulting product was aclear colorless liquid which was found to have a hydroxyl equivalentweight of 68.7, corresponding to an average molecular weight of 206.

The ethoxylated glycerol, 850 g., was returned to the reactor along with250 g. of flaked sodium hydroxide and the charge was heated to 250 F.and stirred under a vacuum of 30 mm. Hg for 2 hours during which time 68g. of water was removed. Methyl chloride was added beginning at 250 F.and was continued for 3 hours at 250-280 F. and 50 p.s.i.g. During thisreaction, 315 g. of methyl chloride was absorbed. The product wasremoved from the reactor and filtered. The sodium chloride byproduct waswashed several times with isopropanol, the isopropanol washings werecombined and the alchohol distilled from the dissolved product. Theproduct was found to have a hydroxide number of 364.

Eight hundred g. of product was returned to the reactor along with 230g. of flaked sodium hydroxide. The charge was heated to 250 F. andstirred under vacuum (30 mm. Hg) for 2 hours. 295 g. of methyl chloridewas then added over a 3 hour period at 250-280 F. and 20-50 p.s.i.g. Theproduct was cooled, discharged and filtered. The salt cake was washedwith several portions of isopropanol. The isopropanol was distilled fromthe dissolved product. The recovered product was a clear colorlessliquid which had a viscosity of 4.5 centistokes at 25 C. The hydroxylnumber of the product was 1 1.0 which indicates 98.6 percent conversionto the trimethyl ether. The calculated molecular weight of the productis 248.

EXAMPLE 2 Preparation of Eth0xylated-Propoxylated Glycerol TrimethylEther In a manner similar to that described in Example l, 396 g. ofglycerol was reacted with 402 g. of ethylene oxide in a first step, and402 g. of propylene oxide in a second step, both steps having beencarried out at 280 F. over a total reaction time of 4 hours. The adductwas a clear pale yellow liquid which had a hydroxyl number of 588,corresponding to an average molecular weight of 286. 870 grams of theadduct was reacted with 220 g. of flaked sodium hydroxide for 3.5 hoursat 250 F. under 30 mm. Hg vacuum. 286 g. of methyl chloride was thenadded at 250 F and 20-50 p.s.i.g. The product was cooled and filtered,and the byproduct sodium chloride was washed with several portions ofisopropanol. After distilling off the isopropanol, the product was foundto have a hydroxyl number of 226. 820 g. of the product was returned tothe reactor and alkylated with 148 g. of sodium hydroxide and 190 g. ofmethyl chloride in the same manner as in Example 1. The product wascooled and filtered, and the salt washed with isopropanol which was thenremoved by distillation. The product was a clear pale yellow liquidwhich had a viscosity of 7.0 centistokes at 25 C. The hydroxyl numberwas 16.1 which indicates a conversion of 97.3 percent to the trimethylether. The calculated molecular weight of the product is 328.

EXAMPLE 3 Preparation of Ethoxylated Sorbitol Tetramethyl Ether To astainless steel pressure reactor is charged 182 g. of sorbitol (l M) and0.3 g. of powdered sodium hydroxide. While heating and melting at about280 F., 176 g. (4 M) of ethylene oxide is slowly added over a period ofabout 3 hours. After vacuum stripping the ethoxylated sorbitol, g. (4.25M) of sodium hydroxide is added and the stirred mixture is heated to 250F. while generated water is removed under a vacuum of about 30 mm. Hg.At the end ofa 3 hour strip, 210 g. (4.2 M) of methyl chloride is addedslowly at 250 F. and 20 to 50 p.s.i. g. When the addition is completed,the product is cooled and filtered. The filter cake is washed with smallportions of isopropanol. The combined filtrate and isopropanol washingsare stripped of the isopropanol. The product has a calculated molecularweight of 4 l 4.

EXAMPLE 4 Preparation of Ethoxylated Pentaerythritol Trimethyl Ether Toa reactor is charged 136 g. (l M) of pentaerythritol, 0.5 g. of sodiumhydroxide and about 200 g. of water to form a slurry. This mixture isheated to 220 F. and first 44 g. (l M) of ethylene oxide is added over a5 hour period. The water is then stripped off and the material istreated with an additional 88 g. (2 M) of ethylene oxide to form the 3mole adduct. Then 125 g. (3.1 M) of sodium hydroxide is added to formthe sodium alcoholate and the mixture is heated at 250 F. whilegenerated water is removed under vacuum at 30 mm. Hg. At the end ofa 4hour strip, 155 g. of methyl chloride (3.1 M) is slowly added at 250 F.and to 50 p.s.i.g. When the addition is complete the product is dilutedwith isopropanol and salt is filtered off. Stripping of the isopropanolgives a product with a calculated molecular weight of 310.

MEASUREMENT OF ACID GAS SOLUBILITIES lN SOLVENTS The suitability of thepolyol ethers of this invention for use as solvents for acid gases wasdemonstrated experimentally in a series of tests wherein thesolubilities of CO and H 5 in a group of solvents were determined.Solubilities were determined by sealing a known volume of solvent in astirred pressure reactor at 100 F. and adding weighed amounts of CO or H8 to the reactor. From the weight of gas added, and the equilibriumpressure obtained, a series of calculations provided the solubility ofthe gas in the solvent at 100 F. and the pressure of the system. Thedata obtained at several pressures when plotted on a graph allowedcalculation of solubility at any desired pressure. The following exampleillustrates the method specifically.

EXAMPLE A To a 500 ml. sample of a solvent in a stirred 2-liter pressurereactor was added liquid carbon dioxide in weighed 5.0 gram increments.After allowing 5 minutes to reach equilibrium, the pressure of thesystem was recorded. This pressure, corresponding to the partialpressure of carbon dioxide, was used to calculate the amount of gas inthe vapor phase. The known weight of carbon dioxide present then allowedthe determination of the weight of dissolved gas. A constant temperatureof 100 F. was maintained during the tests by heating or cooling asnecessary. Solubilities were recorded in terms of grams of carbondioxide per 100 ml. of solvent. A plot of solubility vs. partialpressure permitted comparisons of various solvents at the same pressure.

The solubility of hydrogen sulfide in the solvents was measured byessentially the same method.

In the following Table 1, CO and H 5 solubility in three of the solventsof this invention are recorded along with data for propylene carbonateand polyethylene glycol dimethyl ether, compounds which have been usedcommercially for acid gas removal. The CO solubilities are given at 100p.s.i. partial pressure and the H 5 solubilities are given at 50 p.s.i.partial pressure. The data show these methyl ether derivatives ofpolyether polyols have high solvency for both CO and H 8.

A comparison of some of the physical properties of the compounds of thisinvention with those of propylene car- 5 bonate and polyethylene glycoldimethyl ether are given in Table 2.

Table 2 shows the instant compounds can have an approximately percentlower viscosity than commercial polyethylene glycol dimethyl ether andconsiderable lower vapor pressure than propylene carbonate orpolyethylene glycol dimethyl ether. Low viscosity speeds up absorptionrates, and low vapor pressure decreases solvent losses during flashing.

Foaming is also a problem in actual plant operation of gas purifyingabsorption columns, as this can cause foam-over and reduction ofoperating rates. Tests were therefore run by bubbling air through afritted glass disc into a column of solvent at a controlled rate. It wasfound in this way that the trimethyl ethers of the present inventionfoamed 10 percent less than the polyethylene glycol dimethyl ether and20 percent less than propylene carbonate.

Various changes and modifications of the invention can be made, and, tothe extent that such variations incorporate the spirit of thisinvention, they are intended to be included within the scope of theappended claims.

What is claimed is:

1. A process for removing gaseous acid gases of this group consisting ofcarbon dioxide and hydrogen sulfide from admixture with other gasesselected from the group consisting of hydrogen, nitrogen and loweralkane hydrocarbons, which comprises contacting the gaseous admixture atsuperatmospheric pressure with a solvent comprising a mixture ofpolymethyl ethers of an alkoxylated polyol which has from three to aboutsix hydroxyl groups per molecule, and then removing the dissolved acidgases from the enriched solvent.

2. A process according to claim 1 in which the solvent has an averagemolecular weight from 150 to 800.

3. A process for removing gaseous acid gases of the group consisting ofcarbon dioxide and hydrogen sulfide from admixture with other gasesselected from the group consisting of hydrogen, nitrogen and loweralkane hydrocarbons, which comprises contacting the gaseous admixture atsuperatmospheric pressure with a solvent comprising a polymethyl etherof a polyether polyol, said polyether polyol having the formula 65wherein R is the residue of an organic compound containing therein threeto six active hydrogen atoms present in hydroxyl groups, A is a numberfrom 1 to 6, B is a number from 0 to 5 and A+ B equals 3 to 6, and Yrepresents from 1 to 15 wherein X, Y and Z represent from O to groups orsuch groups in admixture and the sum of X Y Z is l to l5,andAis 1 m4.

5. A process according to claim 4 in which A is l and X, Y and Z areapproximately the same number.

6. A process according to claim 4 in which the solvent is an ethoxylatedglycerol trimethyl ether.

7. A process according to claim 4 in which the solvent is an ethoxylatedpropoxylated glycerol trimethyl ether.

8. A process for removing gaseous acid gases of the group consisting ofcarbon dioxide and hydrogen sulfide from admixture with other gasesselected from the group consisting of hydrogen, nitrogen and loweralkane hydrocarbons, which comprises contacting the gaseous admixture atsuperatmospheric pressure with a solvent comprising a polymethyl etherof a polyether polyol, said polyether polyol having the formula(CH30)FR(YOCH3)D wherein C is a number from 0 to 3, D is a number from 1to 6, E is a number from 0 to 3, F is a number from 0 to 5 and C D E Fequals 3 to 6, Y represents from I to 15 "CIIQCHIO or M CH2 CHO- groupsor such groups in admixture in random or block arrangement, and R is theresidue of an organic compound containing therein three to six activehydrogen atoms present in hydroxyl groups, and then removing thedissolved acid gases from the enriched solvent.

522g? UNITED STATES PATENT OFFICE CERTIFICATE OF connncmoN Patent No.3,653 ,183 Dated Ami-1 4: 1972 Invenwfl Herbert L. Sander s, et a] It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 1, line 62 "y" should be -'Y-; column 2, line 3.8, "U.S. Pat.Nos." should be -United States patents--; column 3, line 21, "and"shouldbe --to-; column 3,

lines 25-29, should be CH O-X-CH H O-X-CH 2 3 2 3 C HO-Y-CH [CHO-Y-CH A"H2O Z"CH3 column 4, line 15 "hydroxide" should be -hydr0xy1; column 6,line 15, "this" should be -the column 7,

lines l5-l, should be c H O-X-CH cn o-x-cn v CHO-Y -CH l HO-Y-CHCH2O-Z-CH3 A" CH2O-Z-CH3 Signed and sealed this 1 8th day of July W972.

(SEAL) Attest EDWARD MQFLETCHER,JR. ROBERT GOTTSCHALK Attesting OfficerCommissionerof Patents

2. A process according to claim 1 in which the solvent has an averagemolecular weight from 150 to
 800. 3. A process for removing gaseous acidgases of the group consisting of carbon dioxide and hydrogen sulfidefrom admixture with other gases selected from the group consisting ofhydrogen, nitrogen and lower alkane hydrocarbons, which comprisescontacting the gaseous admixture at superatmospheric pressure with asolvent comprising a polymethyl etHer of a polyether polyol, saidpolyether polyol having the formula wherein R is the residue of anorganic compound containing therein three to six active hydrogen atomspresent in hydroxyl groups, A is a number from 1 to 6, B is a numberfrom 0 to 5 and A + B equals 3 to 6, and Y represents from 1 to 15groups or such groups in admixture and said polymethyl ether contains atleast three methyl ether groups per molecule, and then removing thedissolved acid gases from the enriched solvent.
 4. A process forremoving gaseous acid gases of the group consisting of carbon dioxideand hydrogen sulfide from admixture with other gases selected from thegroup consisting of hydrogen, nitrogen and lower alkane hydrocarbons,which comprises contacting the gaseous admixture at superatmosphericpressure with a solvent comprising a composition of the formula whereinX, Y and Z represent from O to 15 groups or such groups in admixture andthe sum of X + Y + Z is 1 to 15, and A is 1 to
 4. 5. A process accordingto claim 4 in which A is 1 and X, Y and Z are approximately the samenumber.
 6. A process according to claim 4 in which the solvent is anethoxylated glycerol trimethyl ether.
 7. A process according to claim 4in which the solvent is an ethoxylated propoxylated glycerol trimethylether.
 8. A process for removing gaseous acid gases of the groupconsisting of carbon dioxide and hydrogen sulfide from admixture withother gases selected from the group consisting of hydrogen, nitrogen andlower alkane hydrocarbons, which comprises contacting the gaseousadmixture at superatmospheric pressure with a solvent comprising apolymethyl ether of a polyether polyol, said polyether polyol having theformula wherein C is a number from 0 to 3, D is a number from 1 to 6, Eis a number from 0 to 3, F is a number from 0 to 5 and C + D + E + Fequals 3 to 6, Y represents from 1 to 15 groups or such groups inadmixture in random or block arrangement, and R is the residue of anorganic compound containing therein three to six active hydrogen atomspresent in hydroxyl groups, and then removing the dissolved acid gasesfrom the enriched solvent.